Surface analysis

ABSTRACT

Surfaces are analyzed to determine materials adsorbed thereon by directing a beam of atoms or molecules excited to metastable levels onto the surface. Ions of the adsorbed material which are produced are analyzed, such as by a mass spectrometer.

United States Patent [1 1 Schmidt 1 1 Feb. 12, I974 SURFACE ANALYSIS [75] Inventor: Thomas W. Schmidt, Bartlesville,

Okla.

[73] Assignee: Phillips Petroleum Company, Bartlesyille, Okla.

[22] Filed; Apr. 8, 1971 [211' Appl. No.:'l32,413

[52] US. Cl. 250/413, 250/41.9 SE [51] Int. Cl 301d 59/44 [58] Field of Search 250/495 P, 41.9 SE, 41.3

[56] References Cited UNITED STATES PATENTS 3,415,985 12/1968 Castaing 250/419 SE FOREIGN PATENTS OR APPLICATIONS 1,240,658 8/1960 France 250/419 SE OTHER PUBLICATIONS lon Microprobe Mass Analyzer, Helmut Liebl, Jourmi] of Applied Physics, Vol. 38, No. 13 Dec., 67.

Emission of Charged Particles from Metal Surfaces Under Bombardment by Positive lons, Fogel et 21]., Zhurnal Tekhonicheskoi Fizeki, Vol. 30, No. I, pp. 63-73 Jan., 1960.

Scattering of Low Energy Noble Gas Ions From Metal Surfaces, Smith Journal of Applied Physics, Vol. 38 No. I pp. 340-347.

Primary ExaminerJames W. Lawrence Assistant Examiner-C. E. Church 57 ABSTRACT Surfaces are analyzed to determine materials adsorbed thereon by directing a beam of atoms or molecules excited to metastable levels onto the surface. lons of the adsorbed material which are produced are analyzed, such as by a mass spectrometer.

4 Claims, 2 Drawing Figures Liz It R F GENERATOR VP MASS SPECTROMETER Pmmsn m 1 21914 M ETASTABLE ATOM SOUR CE GENERATOR SURFACE ANALYSIS It is desirable to be able to analyze the surface of a body to determine materials adsorbed thereon and any catalytic effect produced by the body. Such an analysis is useful in the study of catalytic materials and catalytic reactions, for example. Analyses of this type are also useful in the study of selective adsorbents.

In accordance with this invention, a method is provided for studying the surfaces of bodies to determine materials adsorbed thereon. This is accomplished by bombarding the surface with a beam of atoms or molecules which are excited to metastable levels. The excited atoms or molecules ionize and eject materials which maybe absorbed on the surface. The resulting ions are analyzed, such as by the use of a mass spec trometer, to provide information concerning the materials adsorbed and any catalytic effect of the surface. Apparatus useful in carrying out such a method is also provided in accordance with this invention.

In the accompanying drawing, FIG. 1 is a schematic representation of an embodiment of apparatus. constructed in accordance with this invention. P16. 2 is a graphical representation of a typical output signal from a mass spectrometer obtained in the practice of this invention.

Referring now to the drawing in detail and to FIG. 1 in particular, there is showna housing 10, the interior ofwliich is maintained at a reduced pressure. This is accomplished by means of a vacuum pump 11, the inlet of which is connected to the interior of housing by a conduit 12. A metastable atom or molecule source 13 is provided in one end of the housing. A stream of atoms or molecules to be energized is introduced into the source through a conduit 34. A radio frequency generator 16 is connected to the source to establish a radio frequency discharge to excite the atoms or molecules introduced through conduit 13 However, other types of equipment can be employed to produce the excited atoms or molecules, such as equipment employing charge transfer, low voltage are discharge, or cold cathode discharge, for example.

A surface to be studied is positionedin the opposite end of housing 10. In the illustrated embodiment, a metal plate 17 is so positioned by means of support rods 18 and 19 which extend through the end of housing 10. Housing 10 can be provided with an access port 20 to permit plate 17 to be installed. Plates 21 and 22, which have respective apertures 23 and 24 therein, are disposed within housing 10 so that the apertures are in alignment between source 13 and plate 17. This serves to direct a beam of excited atoms or molecules to plate 17.

A conduit 25 communicates with housing 10 at a location adjacent plate 17 so that ions ejected from the surface of the plate are directed through the conduit. Conduit 25 communicates with the inlet ofa mass spectrometer 36. The mass spectrometer serves to detect the masses of the ions produced when the metastable particles impinge on plate 17. A pair of spaced screens 35 and 29 can be disposed in conduit 25. Screen 35 is connected to the contactor of a potentiometer 30, and screen 29 is connected to one end terminal of the potentiometer. A voltage source 31 is connected across the end terminals of potentiometer 30. By suitable adjustment of the contactor of the potentiometer, only ions having energy in a particular range are permitted to pass through screens 35 and 29 to the inlet of the mass spectrometer. The mass spectrometer can be any instrument capable of detecting the masses of the ions introduced therein. The masses of interest can be scanned periodically.

In some operations, it is desirable to maintain plate 17 at an elevated temperature. This can be accomplished by passing current through the plate from a source 32 which is connected ineircuit with the plate by means of a variable resistor 33. If plate 17 is constructed of an electrically conductive material, the plate itself may provide sufficient resistance to generate the desired amount of heat. Otherwise, an electrical heating element can be secured to the back of the plate. If tests are to be conducted at sub-ambient temperatures, a cooling coil can be connected to plate 17.

In the operation of the apparatus of FIG. 1, plate 17 is initially contacted by a material or materials to be studied. The surface is then positioned in housing 10 and subjected to bombardment by the excited atoms or molecules. The generated ions are representative of the material adsorbed on the surface and the state in which the material is adsorbed.

lna specific example of this invention, a platinum surface wasexposed to an atmosphere of ethylene. The surface was then bombarded by helium atoms excited to the metastable state in a radio frequency discharge of about 7 Mhz. The resulting mass spectra of the ejected ions is illustrated in FIG. 2. This is a spectrum representative of ethylene where mass 28 is due to a singly charged ethylene ion. The peaks at mass numbers 14, 26 and 27 represent ethylene fragments. It is thus apparent that some ethylene was adsorbed on the surface, but was not polymerized because no polymer spectrum was obtained.

If chemicals which undergo a reaction in the presence of a catalyst are contacted in the presence of the catalyst,ions representative of the reaction product are observed when the catalyst is bombarded by energized particles in accordance with this invention. The invention is thus quite useful in the direct study of catalytic reactions. Adsorbents can also be investigated to determine materials that are selectively adsorbed. Various molecules and atoms excited to the metastable level can be utilized in accordance with this invention. Numerous examples of such molecules and atoms are described in Molecular Beams, Ross, lnterscience Publishers, New York, I966, page 172.

While this invention has been described in conjunction with presently preferred embodiments, it obviously is not limited thereto.

What is claimed is:

1. The method of analyzing a surface of a body having a material adsorbed thereon, which method comprises the steps of establishing a source of atoms or molecules which are excited to metastable levels, passing a beam of said excited atoms or molecules through a focusing means to impinge onto "the surface to be ana lyzed so as to ionize the material adsorbed on said sur face and eject from said adsorbed material ions representative of the material adsorbed on said surface, passing a beam of the ions so ejected from the adsorbed materail to an analysis means removing from the ions passed to the analysis means ions which have an energy level outside a predetermined range, and analyzing the remaining ions in a mass spectrometer.

2. The method of claim 1 wherein the source is established by subjecting helium to a radio frequency electromagnetic field to produce metastable helium atoms.

3. The method of claim 2 wherein the surface comprises platinum having a hydrocarbon adsorbed thereon.

4. The method of claim 1 wherein the surface comprises a solid material.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent no. 3,792,251 a February 12, 1971,

Thomas W. Schmidt It is certified that error appears in the above-ddentifigd pate nt and that said Letters Patent are hereby correct ed aa ahovm'bolow:

Column 2;, line 57, delete "materail" and insert therefor material 'and column 2, line 57, after "means" insert a comma Signfsd and sealed this 2nd day of July 1974,

(S A Atte s t:

EDWA RD M. FLBLI'CHER,JR. C.MARSHALIJ DAN N Attestlng Offlper Commissioner of Patents 

1. The method of analyzing a surface of a body having a material adsorbed thereon, which method comprises the steps of establishing a source of atoms or molecules which are excited to metastable levels, passing a beam of said excited atoms or molecules through a focusing means to impinge onto the surface to be analyzed so as to ionize the material adsorbed on said surface and eject from said adsorbed material ions representative of the material adsorbed on said surface, passing a beam of the ions so ejected from the adsorbed materail to an analysis means removing from the ions passed to the analysis means ions which have an energy level outside a predetermined range, and analyzing the remaining ions in a mass spectrometer.
 2. The method Of claim 1 wherein the source is established by subjecting helium to a radio frequency electromagnetic field to produce metastable helium atoms.
 3. The method of claim 2 wherein the surface comprises platinum having a hydrocarbon adsorbed thereon.
 4. The method of claim 1 wherein the surface comprises a solid material. 